Multiple problems: Rigged arm & hand with elbow corrective shape key
ORG-forearm.L is not rotating anymore simply because its rotation values are locked (picture n. 1)



In your actual setup the rotation of this bone on his local Z axis has a negative value, so you can get your corrective shape key work with a negative expression (multiplied by 0.5 for having maximum effect roughly at full rotation; picure n. 2).